The invention relates to a method of automatically controlling installations for the preparation of foundry sand, wherein the moisture and compressibility of the sand are measured.
When clay-bonded foundry sand has been used, i.e. cast, it is generally returned to a preparation installation, where the sand is remixed with an adequate quantity of water, bonding agent (e.g. Bentonite), additive (e.g. coaldust, starch) and fresh sand.
The purpose is to proportion the various additives so that the foundry sand has a uniform quality. However, the strain on the foundry sand (e.g. the thermal strain, loss of sand) fluctuates according to each particular production programme, so that old foundry sand with fluctuating properties or different sand characteristics is constantly being recycled to the preparation plant. The aim of a well-functioning preparation process is therefore always to recognise the fluctuations in the old sand and to incorporate the additive with appropriate differentiation during the mixing process.
It is already known to measure the moisture of the foundry sand prior to mixing or in the mixer, and preferably to take the temperature simultaneously. A combined measuring system is also known, where the moisture and density of the sand are measured in the mixer on a capacitative basis at the beginning of the mixing time. It is true that with both these systems premature measurement can be obtained, i.e. measurement prior to mixing or at least at the beginning of the mixing period, so that the moisture content of the sand can still be corrected in a relatively simple way during the mixing period. The first-mentioned, moisture only measurement, however, has the disadvantage that only moisture is measured as the sole variable, and the other fluctuations or variables in the old foundry sand are not taken into account. The second mentioned, combined measuring system does indeed take into account not only the moisture of the sand but also its compressibility and deformability, which again are connected to the piled weight; but it has the disadvantage that the measuring instrument has to be readjusted if there is any change in the characteristic of the sand. It will be appreciated that this necessitates time consuming measures.
Other systems are known, where the compressibility and/or deformability of the sand is ascertained either during the mixing period, by taking samples from the mixer, or after mixing. This measuring method does indeed give direct results for some property of the sand which is important to the processor. The disadvantage, however, is that correction of the addition of water during the mixing period must lengthen the period, since the water is added in stages and each addition has to be mixed in before any new measurement can be taken.
All known systems and processes suffer from the disadvantage that, although it may be possible to establish the compressibility of the sand, the causes of changes in compressibility cannot be ascertained.